Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved front assembly for color tubes having shadow masks of the tensioned foil type in association with a substantially flat faceplate. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the tensioned foil mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high-brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate. As it is under high tension, the tensioned foil mask will dome, but negligibly in comparison with the curved mask. Its relative immunity to doming provides for greater brightness potential while maintaining color purity.
The tensioned foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen consisting of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about one mil thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three beams lands only on its assigned phosphor deposits.
The requirements for a support means for tensioned foil shadow masks are stringent. As has been noted, the foil shadow mask is normally mounted under high tension. The support means should be of high strength so the mask is held immovable; an inward movement of the mask of as little as one-tenth of a mil is significant in expending guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means should have substantially the same thermal coefficient of expansion as the glass, and by its composition, be bondable to glass. Also, the support means should be of such composition and structure that the mask can be secured to it by production-worthy techniques such as electrical resistance welding or laser welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which it is composed should be adaptable to machining or other forms of shaping so that it can be contoured into near-perfect flatness so that no voids between the metal of the mask and the support structure can exist to prevent the positive, all-over contact required for proper mask securement.
A tensioned mask registration and supporting system is disclosed by Strauss in U.S. Pat. No. 4,547,696 of common ownership herewith. A frame dimensioned to enclose the screen comprises first and second space-apart surfaces. A tensed foil shadow mask has a peripheral portion bonded to a second surface of the frame. The frame is registered with the faceplate by ball-and-groove indexing means. The shadow mask is sandwiched between the frame and a stabilizing or stiffening member. When the system is assembled, the frame is located between the sealing lands of the faceplate and a funnel, with the stiffening member projecting from the frame into the funnel. While the system is feasible and provides an effective means for holding a mask under high tension and rigidly planoparallel with the flat faceplate, weight is added to the cathode ray tube, and additional process steps are required in manufacture.
There exists in the marketplace today a color tube which utilizes a tensed shadow mask. The mask is understood to be placed under high tension by purely mechanical means. Specifically, a very heavy mask support frame is compressed prior to and during affixation of the mask to it. Upon release of the frame, restorative forces in the frame cause the mask to be placed under high residual tension. During normal tube operation, electron beam bombardment causes the mask to heat up and the mask tension to be reduced. An upper limit is therefore placed on the intensity of the electron beams used to bombard the screen; this limitation prevents the mask from relaxing completely and thus losing its color selection capability. For a description of this type of tube, see U.S. Pat. No. 3,683,063 to Tachikawa et al.
An avionics color cathode ray tube having ceramic components is described in a journal article by Robinder et al of Tektronix, Inc. A shadow mask is mounted in a ceramic ring/faceplate assembly, with the mask suspended by four springs oriented in the z-axis. Ceramic is also used to form a two-piece X-ray-attenuating body. A flat, high-voltage faceplate is utilized, together with a glass neck flare. (From "A High-Brightness Shadow-Mask Color CRT for Cockpit Displays," Robinder et al. Digest of a paper presented at the 1983 symposium, Society for Information Display.)
A color picture tube having a conventional curved faceplate and correlatively curved, untensed shadow mask is disclosed in Japanese patent No. 56-141148 to Mitsuru Matshusita. The purpose according to a quotation from the abstract is " . . . To rationalize construction and assembly of a tube, by both constituting its envelope from a panel, ceramic shadow mask mounting frame and funnel and integrally forming a surplus electron beam shielding plate to the shadow mask mounting frame."